Thin walled plastic articles and forming same



B. EDWARDS 3,284,553

THIN WALLED PLASTIC ARTICLES AND FORMING SAME Nov. 8, 1966 2 Sheets-Sheet 1 Original Filed May 10, 1961 INVENTOR. Bryanf Edwards BY Nov. 8, 1966 B. EDWARDS 3,284,553

THIN WALLED PLASTIC ARTICLES AND FORMING SAME Original Filed May 10, 1961 2 Sheets-Sheet 2 3 1 19.8 Flgq INVENTOR. Bryam Edwards BY M Arr)

United States Patent Ofilice 3,254,553 i aifented Nov. 8, 1966 3,284,553 THEN WALLED PLASTIC ARTIQLEE; AND FORMING AME Bryant Edwards, Ciarendon Hills, ilL, assignor to lliinois Tool Works, Inc, a corporation of Delaware Original application May 10, 1961, Ser. No. 109,225, now

Patent No. 3,105,607. Divided and this application June 4-, 1963, Ser. No. 285,463

9 Claims. (Cl. 264--89) This is a division of application Serial No. 109,225 filed May 10, 1961, now Patent No. 3,105,607.

This invention relates in general to a method and apparatus for molding of articles and the like from webs of plastic material. More particularly the invention relates to a new and novel method and apparatus for providing in plastic molded containers, lids and the like deep undercuts in their sidewalls and accomplishes this without the use of collets, split molds and the like.

A fabrication of plastic articles from a web of heated sheet plastic material has production advantages in many instances over molding articles by the so-called injection molding technique. One advantage of using the injection molding technique to date has been the ability to produce complex shapes. For example, in injection molding techniques it is possible to get relatively deep undercuts in various port-ions of a molded article. However, in injection molding techniques, there are practical limitations as to the minimum. wall thickness of the molded article which are obtainable and production of very thin walled articles are diflicult if not impossible to obtain.

By using opposed mold members on opposite sides of a heated sheet of web stock, mechanically pre-drawing the material and there the introduction of a vacuum or positive pressure differential across the web, it is feasible to mold plastic articles such as containers, lids and other medium sized articles which have a thickness in the neighborhood of .003 to .035 inch. This method of molding has the advantage of adapting itself to very high speed production and considerably more economical tooling cost as compared with the so-called injection molding technique. Heretofore, the plug assist technique has had the disadvantage of not adapting itself to rapid molding of complex shapes and in particular it has been difiicult to get rapid molding of deep undercuts in the molded .article. In plug assist methods not using split molds, the undercuts have been limited either to the amount of contraction of the thermoplastic material as it cools down from its initial heated state within the mold, or by providing positive stripping means in the mold and using the inherent flexibility of the material to eject slightly oversized parts.

It is possible to mold deep undercuts in the plug assist techniques by the use of complex collets and split molds. However, the use of split molds greatly increases the complexity of the machinery and the length of the production cycle. Complex collets greatly increase the tooling costs and geometry often provents their use. Further, there are various sizes of articles having undercuts therein which made it virtually impossible to design either a split mold or a split or moving port-ion of a collet nature so as to get the desired configuration in the final article.

This invention is concerned with the problem of obtaining deep undercuts in plastic articles without the use of split molds or expanding collets and utilizes the principle that selective portions of a heated web of plastic material may be frozen so as to serve, in a sense, as its own mandrel for the forming of an undercut thereabout. By pre-impinging a portion of a cooled mold member upon selected areas of the heated plastic web, the impinged portion rigidifies; the remaining portions, upon the application of a pressure differential will be caused to move (the heated portions) so as to fold underneath the so-called frozen or rigidified portions (assuming suitable geometry of the remainder of the mold parts).

It is therefore a general object of this invention to provide a novel method and apparatus for forming novel articles from sheet stock plastic material with relatively deep undercuts therein.

It is a further object of this invention .to provide a method .and apparatus for forming undercuts in plastic articles which require no moving collets or split molds.

It is a further object of this invention to provide a method and apparatus which is adaptable to forming variegated complex shapes or patterns in articles while maintainin" the benefits that accrue to molding by the plug assist below molding technique.

It is a further object of this invention to provide a method and apparatus for molding complex shapes with undercuts in plastic articles wherein the contraction rate of the material does not limit the size of the undercut, provides easy stripping of the molded article from the mold, is not limited to certain sizes of articles, and is adapted to fast repetitive economical operation both in production and in initial tooling.

It is another object of this invention to provide thin walled plastic articles having .a wall thickness under .030 inch having relatively deep undercut portions thereon and free from undesirable parting lines.

It is another object of this invention to provide an article of manufacture having undercuts thereon of substantial dimensions which are not limited to the draft angles of the molds and the contraction factor of the material and which has no undesirable mold parting lines.

Still another object of this invention is to provide an article of molded manufacture having more than one undercut surface therein which may be in the same or in different planes and which maybe in the same or different directions.

The novel features that are characteristic of the invention are set forth with particularity in the appended claims. An understanding of the invention is best obtained both as to its organization and mode of operation by reading the following description in conjunction with the accompanying drawings in which:

FIG. 1 is a semidiagrammatic view partially in section showing pertinent portions of upper and lower mold members and an article formed thereby;

FIG. 2 is a top plan view of the article shown in section in FIG. 1;

FIG. 3 is a semidiagrammatic sectional view similar to that shown in FIG. 1 and shows apparatus and an article relating to an alternate embodiment of the invention;

FIG. 4- is a top plan view of the article made in the apparatus shown in FIG. 3;

FIG. 5 is a top view of a portion of the apparatus and article formed therein shown in FIG. 6;

FIG. 6 is a sectional view along line 66 of FIG. 5 relating to another alternate embodiment of the invention;

FIGS. 7 through 12 are sequential semidiagrammatic sectional views illustrating the method and apparatus for forming deep undercuts on articles made from webs of plastic material.

Before discussing the molds and the articles formed thereby os shown in FIGS. 1-6, attention is drawn to FIGS. 7 through 12 which show the sequential operation of the apparatus. The apparatus may be of the general type shown in the United States Patent 2,962,758 and my copending application S.N. 840,604 filed September 17, 1959 Patent No. 3,059,810. Heated sheetstock thermoplastic material 13 such as polyethylene, polystyrene and the like is fed between upper and lower complementary mold means 18 and 20 respectively. Lower and upper annular clamp means 14 and 16 (shown only diagrammatically here but specifically shown and described in Politis Patent 2,962,758) engage opposite sides of the web of material. Each has a diameter larger than the diameter of the complementary lower and upper mold means 18 and 20. Various other clamping arrangements are possible and are contemplated. The lower mold means 18 has an outer mold member 22, which is generally annular in form having a projecting lip portion 24. A clamping cutoff shoulder 26 may be provided on its outer periphery if it is desired to sever completed articles from the plastic web while they are still in the mold. Alternatively, articles can be carried by the web 13 to a severing die removed from the molding area. Outer mold member 22 defines the molding cavity 27. A central projecting mold portion 28 of predetermined diameter may be fixed to the mold member 22 or may be independently actuatable by a separate actuator 30 as shown. The top surface 29 of the central mold portion 28 preferably projects, relative to mold edge portion 24, a distance sufficiently great so that the web material 13 can be brought into contact with portion 28 but without initially contacting mold portion 24 (see FIG. 8) for reasons to be later explained. A relatively movable base mold portion 32 having a depending actuator extension portion 31 is formed with a central bore 33 which surrounds the central mold portion 28. M-old portion 32 is adapted to move relative to mold portion 22 and mold portion 28.

The upper mold means 20 comprises an outer mold member 34 having downwardly projecting portion 36 for cooperation with the clamping cut-off shoulder 36 of the lower mold member 22. A central relatively movable mold portion 40 is mounted in a bore 38 of mold member 34 and is biased to the position shown in FIG. 7 by suitable spring means such as 42. The lower end 41 of central portion 40 extends below mold edge portion 36. Combination cooling means and actuator means 44 is shown diagrammatically and is operable in bore 45 to move central mold portion 40 relative to mold member 34. Means 44 also chills central portion 40.

Mold means 18 and 20 are mounted on platens 19 and 21 respectively (shown diagrammatically) which provide relative axial movement thereof. The actuation means is shown diagrammatically by the arrows 46 and 48.

The operation of the apparatus shown in FIGS. 7 through 12 will now be described. Assume the portions of the apparatus are in the position shown in FIG. 7, i.e. the heated plastic web 13 firmly clamped in clamps 14 and 16 and spaced from both the upper and lower mold means 20 and 18. Upper mold portion 40 is in its downward most position relative to upper mold portion 34 and it will be noted that the end surface 41 thereof extends downwardly beyond the edge portion 36 of member 34. Further it will be noted that central upper mold portion 40 is concentric with and has a greater diameter than the diameter of lower central mold portion 28. The lower mold means 18 has the components therein arranged such that central portion 28 is at or near its uppermost position (as shown in FIG. 7 the top surface thereof being substantially parallel with portion 24 of mold portion 22) and relatively movable lower mold portion 32 is in its lowermost position relative to mold portion 22.

The mold portions sequentially move to the positions shown in FIGS. 7-12. As shown in FIG. 8, actuator means 46 causes relative movement of the upper mold means 20 so that upper center mold portion 40 first engages the web of material 13. It is to be noted that no other portion of either mold means engages the material 13 until relative movement of the mold means causes the end 29 of central portion 28 of the lower mold means to engage the underside of the material. Since upper central mold portion 40 is chilled, the material 13 that is engaged by end 41 of mold portion freezes or is rigidified. The remainder of the clamped heated material 13 remains in its heated state. The two mold means 13 and 20 are now relatively actuated to cause them to assume the positions shown in FIG. 9 such that the material is sealingly clamped between portion 36 of the upper mold means and shoulder 26 of the lower mold means. The relative movement may be accomplished by actuation of the lower actuator means 48.

A pressure differential is now created across the web of material stretched and clamped between the two mold means 18 and 20. This may be done by imposing a positive pressure through bores 45 and 38 as shown diagrammatically by the arrows 50 in FIG. 10 or by creating a vacuum on the underside of the stretched Web through suitable operations in mold means 18 (not shown). The pressure differential causes the web of plastic material to move into engagement with the various portions of the lower mold means 18 to chill same to the configuration shown in FIG. 10. A close review of that figure will show that the pre-frozen portion of the web adjacent end 41 remains rigidified and the heated portion of the web folds underneath and into engagement with the sides of the central mold portion 28. It will be realized that the thicknesses of plastic web material shown in FIGS. 7 through 12 are to be considered semidiagrammatic and in reality the portion immediately adjacent to end 41 of mold portion 40 will be thicker than the remainder of the article (since the heated portions were stretched during the application of the pressure differential).

The mold means 18 and 20 are now actuated towards each other so as to cause a cut-off or severing of the formed article from the web 13 (see FIG. 11). The mold means 18 and 20 are then separated from each other and lower mold portion 32 is actuated relative to lower mold portion 22 so as to cause the article to be stripped from the mold (see FIG. 12) whereupon article removal means 52, here shown in the form of an air jet, is actuated to remove the molded article 54 from the molding area. The clamp means 14-16 are separated and a new heated portion of the web 13 is indexed into position, the clamp means 14-16 close and the apparatus returns to the position shown in FIG. 7 for a new cycle.

The molded article 54 (see FIG. 12) is formed with a central knob 56 having a deep undercut 58 to define a head portion considerably larger than the stem 60. The article 54 may be used as a lid.

The apparatus and article 54a shown in FIGS. 1 and 2 are substantially similar to the apparatus and article just described and similar parts will be given similar reference numbers with the addition of the suffix a. The lower and upper mold means 18a and 20a are shown diagrammatically. Upper central mold portion 40a is formed with a shallow chamfered bore 62 and cooperates with a smaller diameter central concentric projecting mold portion 28a on the lower mold means 18a. Portion 28a is formed with a radially extending tapered shoulder 64 adjacent to the base portion thereof and it will be noted that the end surface 29a projects a considerable distance above lower mold edge portion 24a. The upper portion of central mold portion 28a is slightly tapered as shown. The relatively movable lower mold member 32a may be formed with an annular triangular in cross section ridge 66 which adds strength in the molded article 54a. In some articles, a sinusoidal form is provided (not shown) as an alternative to 66 for purposes of flexibility. The actuation of the upper and lower mold means is in the sequence discussed relative to FIGS. 7 through 12. Thus, upper central mold portion 40a engages the web of material prior to engagement of the material 13 with any other portion of the mold means. Due to the size and configuration of central lower mold portion 28a, the web material is forced up into engagement with the surfaces of the shallow chamfered bore 62 in the end of the upper mold member 40a so as to provide a beveled edge 72 on the article formed in the apparatus. Shoulder 64 of the lower central mold member 28a forms shoulder 74 on article 54a and the triangular ridge 66 forms the offset portion 76 of the article. As can be seen from the sectional view (FIG. 1) a relatively sharp undercut 58a is formed in the article so as to provide a good finger gripping knob. The knob itself projects a considerable distance above the remaining portions of the article. The article 54a is useable as a lid or cannister top on any of a variety of containers. It will be realized that portion 56a of the article 54a, will be thicker than the remaining portions thereof.

The apparatus-and article shown in FIGS. 3 and 4 are substantially similar to the embodiments before discussed and similar portions will be given similar reference numbers with the addition of the suffix 11. Upper mold means 2% is so arranged that the edge portion 36b and central portion 4012 will both contact the web 13 prior to introduction of the pressure differential. Thus the material 13 will be frozen in two discrete areas of the clamped area. The central lower mold portion 28b of the lower mold means 18b may be formed with four (or any desired number) axially aligned thin vane members 76 which project radially from the main body portion of member 28b. The operation of the apparatus is similar to that shown in FIGS. 7 through 12. The article 54b formed in mold means 1815-2012 will have thin ridges or supporting struts 78 adjacent to the stem portion 60b. The use of radial vanes such as 76 on the mold member 28b are particularly desirable when very deep undercuts are made. The reason for this is that it prevents folding of the materials at other than preselected locations when there is an extreme difference in diameter of lower mold portion 28b relative to upper mold portion 4%. It will be noted that two undercuts will be formed in article 54b, one at 58b and one at 80. It is possible by suitable geometry of molds to have these undercuts in the same plane and disposed in the opposite directions as shown. I

The article and apparatus shown in FIGS. 5 and 6 are substantially similar to those aforedis-cussed and similar reference numerals with the addition of the suffix c will be used in the description thereof. The apparatus and article shown in FIGS. 5 and 6 illustrate that rather complex shapes may be formed by the instant process. For example, the upper relatively movable central mold member 40c may be formed with a plurality of scallops 84 in the peripheral edge thereof. Thus when portion 400 engages the web of material, only the part that is engaged freezes and the material will fold thereunder as shown in FIG. 6 so as to form scallops 86 in the article 54c.

In FIGURES 1 through 6 the upper and lower mold means 20 and 18 are shown with semidiagrammatic passageways 68 and 70 for entry of a coolant to the molds. Thus it will be seen that contact of the web 13 with the various mold members causes freezing thereof and by selectively contacting portions of the web by a mold member or portion of a mold member selective portions can be frozen to serve as a mandrel for moving of the material into a deep undercut. There is no dependence in mold design upon the contraction rate of the material for stripping purposes. Stripping of the article is easily accomplished by the instant method and the apparatus may be operated in a fast repetitive and economical manner. Since there is no requirement for split molds or moving collets, no mold parting lines appear in the area of the undercut on the article.

Although specific embodiments of the invention have been shown and described, it is with full awareness that many modifications thereof are possible. The invention, therefore, is not to be restricted except insofar as is necessary by the prior art and by the spirit of the appended claims.

What is claimed as the invention is:

1. A method of molding undercuts in articles made from sheet stock plastic material comprising heating said material to afford ready stretching thereof and disposing said heated material between first and second opposed cooled mold members, one of said mold members having a sheet engaging portion of a first size, the other of said mold members having an initial sheet engaging portion of smaller size than said first size, freezing a portion of the plastic material below the temperature affording ready stretch-ing thereof by engagement with said first size portion of said one mold member, relatively moving the smaller size initial sheet engaging portion of the other mold member and said sheet engaging portion of said first size of said one mold member into proximity to each other with the frozen portion of sheet material therebetween and app-lying a pressure differential to opposite sides of the unfrozen port-ions of the sheet so as to cause the unfrozen heated portions to stretch and fold under the frozen portions to provide an undercut on an article.

2. Apparatus for molding undercuts in articles from a heated web of plastic material comprising clamp means for clamping said web, first and second opposed mold means disposed on opposite sides of the heated web of material, said first mold means having a first Web engaging portion of a first size, said first web engaging portion having a cooled portion of a temperature sufficiently low so as to cool a portion of the web upon contact therewith so as to prevent stretching of said portion, said second mold means having a second portion disposed in generally aligned and opposed relation to said first portion and of a size smaller than said first portion, and means for creating a pressure diffeerntial across said web portion after contact of said cooled first web engaging portion of said first mold means to cause said remaining heated portion of said web to fold under the cooled portion thereof and into engagement with said second portion of said second mold means to provide a precisely dimensioned undercut on an article.

3. The apparatus set forth in claim 2 wherein the first and second portions of the first and second mold means are curvilinear.

4. A method of forming undercuts in articles made from sheet stock plastic material comprising, heating at least a portion of the plastic material to afford ready stretching thereof, freezing at least a portion of the plastic material in an area adjacent to the heated material portion below the temperature affording ready stretehing thereof, and moving at least part of the heated material portion to one side of the frozen mraterial portion While supporting an area of said frozen material portion with-in the outer peripheral portions thereof to provide an undercut on an article.

5. The method as defined in claim 4 wherein discrete, spaced areas of the plastic material are frozen below the temperature affording stretching thereof in an area adjacent to the heated material portion.

6. The method as defined in claim 4 'w heerin a predetermined tarea of the heated material portion is frozen below the temperature affording ready stretching thereof so as to completely surround the frozen material portion.

7. The method as defined in claim 4 including the step of moving the heated material portion between opposed mold members of predetermined different size, the largest mold member capable of freezing part of the heated material portion, and the smaller mold member capable of 'being moved into engagement with a subportion of the frozen material portion to limit the amount of subsequent stretching of the heated material portion.

8. Apparatus for molding undercuts in articles from sheet stock plastic material comprising means for heating at least a portion of the plastic material to afford ready stretching thereof, means for freezing at least a portion of the plastic material in an area adjacent to the heated material portion below the temperature affording ready stretching thereof, and means for moving at least a part of the heated material portion to one side of the frozen 7 8 material portion While supporting an area of said frozen 2,484,656 10/1949 Sikka et a1 1819 XR material portion Within the outer peripheral portions 2,550,062 4/1951 Glasner et a1. 113-38 thereof to provide an undercut on an article. 3,004,288 10/ 1961 G d 9. The apparatus as defined in claim 8 wherein the 3 050 773 3 1952 Hagen 264 29 XR means for moving at least part of the heated material por- 5 tion comprises means for establishing a pressure differ- FOREIGN PATENTS ential on opposite sides of the plastic material.

References Cited by the Examiner UNITED STATES PATENTS 10 ROBERT F. WHITE, Primary Examiner.

1972789 9/1934 Newkirk 18 "19 XR M. R. DOWLING, Assistant Examiner. 2,348,905 5/1944 Hopfield 65-107 XR 688,886 6/1964 Canada. 

4. A METHOD OF FORMING UNDERCUTS IN ARTICLES MADE FROM SHEET STOCK PLASTIC MATERIAL COMPRISING, HEATING AT LEAST A PORTION OF THE PLASTIC MATERIAL TO AFFORD READY STRETCHING THEREOF, FREEZING AT LEAST A PORTION OF THE PLASTIC MATERIAL IN AN AREA ADJACENT TO THE HEATED MATERIAL PORTION BELOW THE TEMPERATURE AFFORDING READY STRETCHING THEREOF, AND MOVING AT LEAST PART OF THE HEATED MATERIAL PORTION TO ONE SIDE OF THE FROZEN MATERIAL PORTION WHILE SUPPORTING AN AREA OF SAID FROZEN MATERIAL PORTION WITHIN THE OUTER PERIPHERAL PORTIONS THEREOF TO PROVIDE AN UNDERCUT ON AN ARTICLE. 